1. Field of the Invention
The present invention relates to graphitic polyhedral crystals that take the form of carbon nanotubes, carbon whiskers or nanorods and have a polyhedral cross section, a variety of uses thereof, and methods for their production from glassy carbon starting materials.
2. Description of the Related Art
Both graphite whiskers (R. Bacon, J. Appl. Phys. 31, 283-290 (1960)) and carbon nanotubes (S. Iijima, Nature 354, 56-58 (1991)) represent unusual forms of carbon based on the distortion of graphene sheets. Conventional graphite forms hexagonal plate-like crystals with a very weak bonding between graphene layers. Graphite whiskers, in which a graphene sheet rolls into a scroll, provide a benchmark against which the performance of carbon fibers is compared. The discovery of carbon nanotubes demonstrated the possibility of making strong graphite crystals consisting of coaxial closed tubes and challenged the position of whiskers as the strongest material known.
Polyhedral carbon nanotubes have been reported (P. M. Ajavan, in Carbon Nanotubes: Preparation and Properties T. W. Ebbesen, Ed. (CRC Press, Boca Raton, Fla., 1997) pp. 111-138; M. S. Dresselhaus et al, Science of Fullerens and Carbon Nanotubes (Academic Press, 1996); S. Iijima et al, Phys.Rev.Lett. 69, 3100-3103 (1992); S. Iijima, MRS Bull. 19, 43-49 (1994)), but did not receive much attention. To date, only 5 and 6 membered polyhedral structures have been reported. A theoretical treatment of the electronic properties of polygonized carbon nanotubes has also appeared, but included only ab initio calculations of the electronic properties based upon hybridization effects, particularly in zigzag nanotubes having high curvature near the edges of the polygonal cross-section prism (J.-C. Charlier et al, Phys. Rev. B, 54(12), R8377-R8380 (1996)). Faceting of vapor-grown carbon fibers occurs after heating to 3000xc2x0 C. (M. Endo et al, in Carbon Nanotubes T. W. Ebbesen, Ed. (CRC Press, Boca Raton, 1997) pp. 35-110). Of all carbon fibers, those are the closest to crystalline graphite in crystal structure and properties. However, faceted carbon whiskers have not been reported. Partial graphitization and formation of polygons was observed after heat treatment of carbon black at 2800xc2x0 C. (Endo et al). Polyhedral nanoparticles (5-30 nm) made of concentric layers of closed graphene sheets and having a nanoscale cavity in the center were produced at very high temperatures in arc plasma (D. Ugarte, in Carbon Nanotubes M. Endo, S. Iijima, M. S. Dresselhaus, Eds. (Pergamon, Oxford, 1996) pp. 163-1679). They provided further evidence that non-planar graphite crystals can exist. However, transformation of carbon polyhedra to onions under electron irradiation suggested their instability. No other polyhedral carbon structures have been reported so far.
Carbon whiskers and nanotubes have received a high level of attention in recent years, for their use as nanometer-scale microscopy probes (Lieber et al, U.S. Pat. No. 6,159,742; Baldeschwieler et al, U.S. Pat. No. 5,824,470), as materials in thermal composites, reinforcement composites and magnetic particle recording media (Nolan et al, U.S. Pat. Nos. 5,780,101 and 5,965,267), as field emission tips in field emission devices and flat panel displays (Park et al, U.S. Pat. No. 6,019,656), in the production of electrodes for electrochemical capacitors (Tennent et al, U.S. Pat. No. 6,031,711), as emitters in cold cathode emitter structures (Chuang et al, U.S. Pat. No. 6,062,931), as functional elements in MEMS devices (Mancevski, U.S. Pat. No. 6,146,227), as quantum wires in a quantum wire switch (Flory et al, U.S. Pat. No. 5,903,010), in the production of miniaturized solenoids for the production of strong magnetic fields using weak current on a small scale (Miyamoto, U.S. Pat. No. 6,157,043), to produce micro or nanoscale electrical contact probes (Bahns et al, U.S. Pat. No. 6,020,747), as well as others.
However, one difficulty in the previously isolated nanotubes and carbon whiskers has been the ability to manipulate the circular cylindrical shapes of submicrometer diameter readily, or in the case of the polygonal nanotubes previously found, to avoid the instability noted above. A further difficulty found in working with nanotubes or carbon whiskers having a circular cross section is the tendency of the cylinder to undergo separation and telescoping of the various layers of graphene sheet walls.
Accordingly, one object of the present invention is to provide graphitic polyhedra structures that have increased stability and improved manipulability relative to circular cylindrical structures.
A further object of the present invention is to provide polyhedral cross-section nanotubes or carbon whiskers having significantly increased strength and structural integrity relative to those with circular cross sections.
A further object of the present invention is to provide a method for the production of such graphitic polyhedra, particularly polyhedral cross-section nanotubes and carbon whiskers.
A further object of the present invention is to provide a nanotube or carbon whisker having a polyhedral cross-section and having a twist along the long axis to provide even higher structural integrity and strength.
These and other objects of the present invention have been satisfied by the discovery of an isolated graphitic polyhedral crystal comprising graphite sheets arranged in a plurality of layers to form an elongated structure having a long axis and a diameter and having 7 or more external facets running substantially the length of the long axis, a method for the isolation of such graphitic polyhedral crystals and their use in a variety of nanoscale devices.